Inheritance: building one class on another

What you will be able to do

After this lesson you can

• build a child class on a parent class,
• explain what the child gets, and what it adds,
• use isinstance and issubclass, and
• call the parent's __init__ with super().

A kind of a kind

A dog is an animal. A cat is an animal. All animals have a name and can eat. Dogs can also fetch, and cats can also climb. If you write a class for each kind, you would write name and eat again and again. There is a better way: write what all animals share once, in a class Animal, and build Dog and Cat on it. This is called inheritance.

Predict first. Dog has no __init__ and no eat. Will Dog("Rex").eat() work? Then run it.

• class Dog(Animal): writes the parent in brackets. Animal is the parent (or base class, or superclass). Dog is the child (or subclass).
• A Dog gets everything that Animal has. Dog("Rex") uses the inherited __init__, and sets the name. rex.eat() and rex.speak() are inherited too.
• fetch is new: only dogs have it. A child adds to the parent.
• isinstance(rex, Dog) is True. isinstance(rex, Animal) is also True: a dog is an animal. isinstance(Animal("x"), Dog) is False: not every animal is a dog. The relation goes one way.
• issubclass(Dog, Animal) asks the same about two classes.
• Dog.__mro__ is the list of classes that Python searches, in order, for a method: first Dog, then Animal, then object, the class under every class.

So the rule: Python looks for a method or attribute in the object, then in its class, and then up through the parents.

The child's own set-up: super()

Suppose that a Dog needs more data: a list of tricks. It writes its own __init__. But then the __init__ of Animal is not run, unless the child asks for it. The tool is super(): "the parent".

What if the child forgets?

Why inheritance?

• Write once. The shared code lives in one place. A fix in Animal fixes every child.
• A family. Code that works for an Animal works for every kind of animal, as the next lesson shows.
• Small differences. A child only writes what is different.

Both objects answer isinstance(shape, Shape) with True: a square is a shape. That is the test for inheritance: "is a".

When not to use it

Inheritance says "is a". A Dog is an Animal. But a Car is not an Engine. A car has an engine. If the "is a" sentence sounds wrong, do not use inheritance. You will see the other tool, composition, in the fourth lesson.

Common mistakes

• Forgetting super().__init__ in a child with its own __init__.
• Passing the wrong arguments to the parent's __init__.
• Using inheritance when the sentence "is a" is false.
• A deep family tree. Three levels are plenty. More is hard to follow.
• Copying the parent's code into the child instead of inheriting it.

Practice

Start with two quick questions. Each New numbers gives you a fresh one, so you can practise as long as you like.

Then the labs and the problems:

1. Warm-up. Open the problem Who is what? and use isinstance on a small family.
2. Build. Open the lab A child class. You build a class on another and the checker tries it.
3. Stretch. Open the problem Fix the child and find why a child object has no name.

Remember these

• class Child(Parent): builds a child on a parent. The child gets everything and adds its own.
• isinstance(x, Class) is true for the class and for the classes built on it.
• A child with its own __init__ calls super().__init__(...).
• Use inheritance only for "is a".

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. What does class Dog(Animal): mean?

2. Is isinstance(Dog("Rex"), Animal) true or false?

3. What goes wrong if a child writes __init__ and does not call super().__init__?